Spin-Axis Dynamic Locking

Abstract

The all-electrical realization of highly spin-polarized charge currents and their efficient conversion into pure spin currents remains a fundamental challenge in spintronics. Here, we report a spin-axis dynamic locking (SADL) effect in altermagnets that pins the spin polarization to the crystalline axes: an in-plane electric field along one principal axis drives a fully spin-up charge current, whereas along the orthogonal axis, it generates an equal spin-down current. Consequently, applying an electric field diagonally yields a transverse pure spin current with 100% charge-to-spin conversion efficiency. Mechanistically, SADL originates in a distinctive tent state characterized by alternating spin-split flat bands and orthogonal Fermi-surface lines. High-throughput first-principles screening confirms SADL in broad materials (e.g., 2D Cr2WSe4 monolayer and a synthesized three-dimensional compound, (BaF)2Mn2Se2O). Our work thus opens a route to ultra-low-power, reconfigurable spintronic devices where the spin states are governed solely by electric field orientation.

Figures (3)

• Figure 1: Lattice model, electronic structure, and SADL effect. (a) Schematic of the Néel AFM lattice, with the primitive unit cell shaded in green. Arrows indicate dominant nnn hopping pathways. (b) 3D representation of the tent-state band structure. (c) Altermagnetic bands along high-symmetry paths, showing $d$-wave spin splitting. (d) Spin-resolved DOS, highlighting van Hove singularities. (e) Energy-dependent conductivity and polarization for charge currents along the principal crystal axes. (f-h) SADL effect under an in-plane field: (f) $\bm{E} \parallel [100]$ drives a longitudinal, fully spin-up-polarized charge current; (g) $\bm{E} \parallel [010]$ drives a longitudinal, fully spin-down-polarized charge current; (h) $\bm{E} \parallel [110]$ yields a non-polarized longitudinal charge current and a transverse pure spin current.
• Figure 2: Crystal structure, tent state, and SADL effect in Cr$_2$WSe$_4$. (a) Top and side views of the monolayer structure. (b) Electronic band structure with an enlarged view of the tent state (inset). (c) Spin-polarized charge density distribution. (d) Alternating spin splitting, wherein the energy ordering of spin-up and spin-down states reverses between symmetry-related crystal axes, demonstrating d-wave altermagnetic characteristics. (e, f) Wave function isosurfaces corresponding to the states labeled wave-1 (spin-down) and wave-2 (spin-up) in (b), respectively, highlighting their 1D spatial extent. (g) Energy-dependent axial conductivity and polarization. Inset: orthogonal Fermi-surface lines of the low-energy conduction bands at 0.5 eV.
• Figure 3: SADL effect in the 3D crystal (BaF)$_2$Mn$_2$Se$_2$O. (a) Crystal structure. (b) The structure of MnO layer. (c) Brillouin zone. (d) Electronic band structure. (e) Illustration of SADL under gate voltage tuning: spin-up carrier transport along the $x$-direction and spin-down carrier transport along the $y$-direction. (f) Fermi surfaces of the spin-polarized valence band ($U_g < 0$) and conduction band ($U_g > 0$). (g) Energy-dependent conductivities along different directions.